1. Field of the Invention
The present invention relates to a process for preparing a cation-exchange membrane which has excellent electrical characteristics and cation selective permeability, chemical resistance, heat resistance and mechanical strength. More particularly, it relates to a process for preparing a cation-exchange membrane which is suitable as a diaphragm for electrolysis of an alkali metal halide.
2. Description of the Prior Art
Cation-exchange membranes have been used in various industries because of excellent electrical characteristics and cation selective permeability. For example, the cation-exchange membranes have been used as diaphragms for electric dialysis by combining it with an anion-exchange membrane or the other membrane, in a concentration of sea water for producing sodium chloride, a desalting of brine or sea water for producing fresh water; a recovery of useful metal salt from a waste water in a metal plating; a treatment of drainage; a purification of juice; a desalting of powdery milk; a treatment of waste solution containing radioactive material; a concentration of uranium and the other various purposes.
In the application of the cation-exchange membrane to these usages, various characteristics are required. The most important characteristics are the cation selective permeability and the durability.
Various efforts have been made for improving the ion selective permeability and the durability since the ion-exchange membrane has been found.
As the result, cation-exchange membranes made of styrenedivinylbenzene crosslinked polymer having sulfonic acid groups have been developed as the cation-exchange membrane, and the cation-exchange membranes having enough selective permeability and durability in many usages have been proposed.
However, the conditions using the ion-exchange membranes have been severe as the usages for the treatment of drainage and waste solution or the electrolysis of sodium chloride. Further improvement of the durability of the membrane has been required. The selective permeability of the ion-exchange membranes has not been satisfactory in some usages.
The conventional cation-exchange membranes have high resistance to the permeation of most of anions however, they have disadvantages to be remarkably high mobility of anions to the electrolyte aqueous solution containing hydroxyl group ions in comparison with the other cases.
The phenomenon is caused because the permeation of hydroxyl group ions can not be effectively prevented as the mobility of hydroxyl group ions in an aqueous solution is remarkably high in comparison with the other anions.
When the cation-exchange membrane is used in the condition containing hydroxyl group ions, for example it is used as a diaphragm for electrolysis of sodium chloride, the current efficiency is lowered disadvantageously because of the phenomenon.
Accordingly, it has been required to develop a cation-exchange membrane having high durability and high resistance to permeation of hydroxyl group ions in these usages of the cation-exchange membrane.
The inventors have studied to develop the cation-exchange membrane for satisfying the requirement of the durability and the permeation of hydroxyl group ions.